Introduction to State Diagrams State Charts in UML

1
Introduction to State Diagrams / State Charts in
UML

• CS320 Fundamentals of Software Engineering
• Slides adapted from Matthias Sand

2
State Machines Overview

• Basic State Machine Concepts
• State Diagrams / State Charts and Objects
• Advanced Modeling Concepts
• Wrap Up

3
Automata

• A machine whose output behavior is not only a
  direct consequence of the current input, but of
  some past history of its inputs
• Characterized by an internal state which
  represents this past experience

4
State Machine (Automaton) Diagram

• Graphical rendering of automata behavior

Lamp On
Lamp Off
5
Outputs and Actions

• As the automaton changes state it can generate
  outputs

6
Extended State Machines

• Addition of variables (extended state)

ctr Integer
7
A Bit of Theory

• An extended state machine is defined by
• a set of input signals (input alphabet)
• a set of output signals (output alphabet)
• a set of states
• a set of transitions
• triggering signal
• action
• a set of extended state variables
• an initial state designation
• a set of final states (if terminating automaton)

8
Basic UML Statechart
top state
State
Initial pseudostate
Trigger
Ready
Transition
/ctr 0
stop
Done
Final state
Action
9
What Kind of Behavior?

• In general, state machines are suitable for
  describing event-driven, discrete behavior
• inappropriate for modeling continuous behavior

10
Event-Driven Behavior

• Event a type of observable occurrence
• interactions
• synchronous object operation invocation (call
  event)
• asynchronous signal reception (signal event)
• occurrence of time instants (time event)
• interval expiry
• calendar/clock time
• change in value of some entity (change event)
• Event Instance an instance of an event (type)
• occurs at a particular time instant and has no
  duration

11
The Behavior of What?

• In principle, anything that manifests
  event-driven behavior
• Guarded Sequence Diagrams are used in UML for
  modeling continuous behavior
• In practice
• the behavior of individual objects
• object interactions
• The dynamic semantics of UML state machines are
  currently mainly specified for the case of active
  objects

12
Basic State Machine Concepts State Diagrams /
Statecharts and Objects Advanced Modeling
Concepts Wrap Up
13
Object Behavior

• Simple server model

InitializeObject
Handling depends on specific request type
Wait forRequest
HandleRequest
TerminateObject
14
Object Behavior and State Machines

• Direct mapping

15
Basic State Machine Concepts Statecharts and
Objects Advanced Modeling Concepts Wrap Up
16
State Entry and Exit Actions

• A dynamic assertion mechanism

entry/lamp.on()
exit/lamp.off()
17
Order of Actions

• Exit actions prefix transition actions
• Entry action postfix transition actions

Resulting action sequence printf(exiting) pr
intf(to off) lamp.off()
printf(exiting) printf(needless) lamp.off()
18
Internal Transitions

• Self-transitions that bypass entry and exit
  actions

Internal transition triggered by an off event
off/null
19
State (Do) Activities

• Forks a concurrent thread that executes until
• the action completes or
• the state is exited through an outgoing transition

do activity
do/while (true) alarm.ring()
20
Guards

• Conditional execution of transitions
• guards (Boolean predicates) must be side-effect
  free

21
Static Conditional Branching

• Merely a graphical shortcut for convenient
  rendering of decision trees

value gt 200 /sell
value lt 100 /reject
(value gt 100) (value lt 200) /sell
22
Dynamic Conditional Branching

• Choice pseudostate guards are evaluated at the
  instant when the decision point is reached

bid /gain calculatePotentialGain(value)
gain gt 200 /sell
gain lt 100 /reject
(gain gt 100) (gain lt 200) /sell
Dynamic Choice point
23
Hierarchical State Machines

• Graduated attack on complexity
• states decomposed into state machines

LampFlashing
24
Stub Notation

• Notational shortcut no semantic significance

LampFlashing
flash/
FlashOn
on/
FlashOff
25
Group Transitions
Default transition to the initial pseudostate

• Higher-level transitions

LampFlashing
Group transition
26
Completion Transitions

• Triggered by a completion event
• generated automatically when an immediately
  nested state machine terminates

completion transition (no trigger)
27
Triggering Rules

• Two or more transitions may have the same event
  trigger
• innermost transition takes precedence
• event is discarded whether or not it triggers a
  transition

LampFlashing
FlashOn
off/
FlashOff